This invention relates to the field of multistage amplifiers and, more particularly, to a novel compensation topology for multistage amplifiers having high gain as well as high bandwidth.
High-gain and high-speed amplifiers are vital in modern analog electronic circuits and are used in a wide range of applications. The increasing tendency towards low-voltage designs, especially as many devices and integrated circuits are made to smaller dimensions, causes significant problems in amplifier design.
A particular problem is that as the power supply voltage is scaled down in the design, the threshold voltage does not necessarily scale down in the same way. For an operational amplifier in such a situation conventional vertical gain enhancement techniques (cascoding) are no longer suitable for low-voltage applications and instead horizontal gain enhancement techniques (cascading) are employed. Typically, modern cascaded amplifiers comprise three amplifier stages. However, with three cascaded amplifiers the stability of the amplifier and its bandwidth are both limited using existing frequency compensation techniques.
To achieve stability at high frequencies in multistage amplifiers, a stabilization topology called Nested Miller Compensation (NMC) was proposed by R. Eschauzier and J. Huijsing, xe2x80x9cFrequency Compensation Techniques for Low Power Operational Amplifiersxe2x80x9d, Boston, Mass., Kluwer, (1995). The main drawback of this technique is that the Miller capacitors in the feedback path introduce zeroes in the Right Hand complex S-Plane (RHP) degrading stability of the amplifier at high frequencies. The position in the S-Plane of one of these zeroes is directly related to the transconductance of the final stage amplifier. As the transconductance of the final stage amplifier changes due to changes in the load also the positions of the zeroes with respect to frequency changes.
To overcome the drawback of the NMC Fan You and S. H. Embabi disclosed in xe2x80x9cMultistage Amplifier Topologies with Nested Gm-C Compensationxe2x80x9d, IEEE J. Solid State Circuits, Vol. 32, pp 2000-2011, (1997) the nested Gm-C Compensation (NGCC) topology to cancel out the zeros introduced by the Miller capacitors. However, it is not possible to use this technique for multistage amplifiers having high gain as well as high bandwidth, because it only cancels the zeros introduced by the Miller capacitors. For example, in case of a three stage amplifier the transfer function has three poles, which provide substantial difficulties in the design of a three stage amplifier having high gain as well as high bandwidth.
To achieve higher bandwidth a feed forward path has been introduced into the existing NMC compensation circuits. The feed forward path introduces a zero in the transfer function, which is positioned such that it cancels out one of the non-dominant poles in the transfer function. This topology is known as Multipath Nested Miller Compensation (MNMC) and is disclosed in:
R. Eschauzier and J. Huijsing, xe2x80x9cFrequency Compensation Techniques for Low Power Operational Amplifiersxe2x80x9d, Boston, Mass., Kluwer, (1995);
R. Eschauzier, L. Kerklaan, and J. Huijsing; xe2x80x9cA 100-MHz 100-dB Operational Amplifier with Multipath Nested Miller Compensation Structurexe2x80x9d, IEEE J. Solid-State Circuits, Vol. SC-27, pp. 1709-1717, (1992); and,
K. Langen, R. Eschauzier, and J. Huijsing, xe2x80x9cA 1 GHz Class-AB Amplifier with Multipath Nested Miller Compensation for 76 dB gainxe2x80x9d, IEEE J. Solid State Circuits, Vol. SC-32, pp. 488-498, (1997).
However, this compensation topology also suffers from the same drawbacks as the NMC topology and does not provide reliable operation of multistage amplifiers having high gain as well as high bandwidth.
It is, therefore, an object of the invention to provide a compensation topology allowing stable operation of high gain, high bandwidth multistage amplifiers. In particular, it is an object of the invention to provide a compensated three stage amplifier having high gain as well as high bandwidth.
In order to overcome the drawbacks of the prior art the compensation technique according to the invention introduces circuit elements to cancel the zeros introduced by the Miller capacitors as well as one of the non-dominant poles in the transfer function of a three stage amplifier. The final transfer function of a such compensated cascaded three stage amplifier has then only two poles. Such a two pole system is much more desirable for designing a stable cascaded amplifier having high gain as well as a wide operating bandwidth, because it substantially facilitates the design of a stable operating three stage amplifier.
According to the invention there is provided a compensated three stage amplifier comprising:
a first, a second and a third cascaded gain stage for amplifying an input signal and providing an output signal in dependence thereupon, each gain stage comprising an input port and an output port, wherein the input signal is received at the input port of the first gain stage and wherein the output signal is provided by the output port of the third gain stage;
a first feedback loop provided from the output port of the third gain stage to the output port of the second gain stage;
a second feedback loop provided from the output port of the third gain stage to the output port of the first gain stage;
a first feed forward transconductance stage extending from the input port of the first gain stage to the output port of the third gain stage, wherein the first feed forward transconductance stage is for canceling one of the zeros introduced by the first and second feedback loop in the transfer function of the three stage amplifier;
a second feed forward transconductance stage extending from the output port of the first gain stage to the output port of the third gain stage, wherein the second feed forward transconductance stage is for canceling another one of the zeros introduced by the first and second feedback loop in the transfer function; and,
a third feed forward transconductance stage extending from the input port of the first gain stage to the output port of the second gain stage, wherein the third feed forward transconductance stage is for canceling one of two non-dominant poles in the transfer function resulting in a transfer function of the three stage amplifier having only two poles.
According to the invention there is provided a compensated three stage amplifier comprising:
a NMC and NGCC compensated cascaded three stage amplifier for receiving an input signal and for providing an amplified output signal in dependence thereupon; and,
further compensation means, wherein the further compensation means is designed such that it cancels one of two non-dominant poles in the transfer function of the NMC and NGCC compensated cascaded three stage amplifier for providing a transfer function having only two poles.
According to the invention there is also provided a compensated three stage amplifier comprising:
a NMC and NGCC compensated cascaded three stage amplifier for receiving an input signal and for providing an amplified output signal in dependence thereupon; and,
a non-inverting gain stage, wherein the non-inverting gain stage is for canceling one of two non-dominant poles in the transfer function of the NMC and NGCC compensated cascaded three stage amplifier for providing a transfer function having only two poles.
According to the invention there is further provided a method for compensating a cascaded three stage amplifier comprising the steps of:
providing a NMC and NGCC compensated cascaded three stage amplifier for receiving an input signal and for providing an amplified output signal in dependence thereupon;
providing further compensation means; and,
using the further compensation means for cancelling a non-dominant pole in the transfer function of the NMC and NGCC compensated cascaded three stage amplifier in order to provide a transfer function having only two poles.